Friday, April 16, 1999 Published at 12:25 GMT 13:25 UKSci/TechKeeping tabs on EarthAn artist's impression of Landsat 7 (Nasa)They must be among the most successful of all the satellites that have been put into orbit. The Landsat series of spacecraft have been sending back invaluable information about the health of our planet since 1972.

Now, the very latest version of the satellite - Landsat 7 - is ready to carry on the good work. Just launched onboard a Delta II rocket, it will fly over and document the condition of the entire globe every 16 days.

On each of those days, it will collect 250 scenes, every one of them containing enough digital data to fill a powerful home computer's hard drive.

This information can be used to determine the health of crops and other vegetation by monitoring water content in the soil. It can keep tabs on natural disasters and monitor population changes in metropolitan areas.

The importance of the Landsat series has increased in tandem with the concerns about the global environment.

Global deforestation

Landsat is particularly useful for monitoring problems like global deforestation and fire damage. It can see the ebb and flow of glaciers and assess water quality in lakes.

This latest version of Landsat, circling 705 kilometers (438 miles) above the Earth, is equipped with updated technology. Its brain is the Enhanced Thematic Mapper Plus (ETM+). The instrument is a passive sensor, a type of remote-sensing instrument that measures solar radiation reflected or emitted by the Earth.

The instrument has eight bands sensitive to different wavelengths of visible and infrared radiation.

Needless to say, Landsat 7 is far more accurate than its predecessors. The new data will be extremely detailed - it can "see" features on the planet as small as 30 metres. This is so good that scientists who, for example, study volcanoes can actually produce maps of lava flows with pinpoint accuracy.

Landsat is a joint program of Nasa and the US Geological Survey. A data centre in Sioux Falls, South Dakota, will be the primary receiving station for all the information collected. Nasa has promised to make the data gathered by Landsat more widely available.